1. Field of the Invention
The present invention relates to a method for sterilizing packaging materials by using high voltage pulses power source and a device therefore. More specifically, it relates a method for sterilizing packaging materials for packaging materials such as liquids, solid bodies, or combinations of them such as foods, medical supplies, Chinese medicines, cosmetics, feeds, and fertilizers, especially, and to a method for sterilizing packaging materials for packaging foods by using high voltage pulses and a device thereof.
2. Detailed Description of the Related Art
Conventionally, as methods for sterilizing the packaging materials, reagent sterilization using an aqueous solution of peroxide, gas sterilization using ethylene oxide, ultraviolet sterilization, heat sterilization, or the like have been carried out in general. Moreover, many methods of sterilization (under vacuum or under normal pressure) using plasma classified as the same technology as this invention have been disclosed in the patent publications.
However, the reagent sterilization has a problem that sterilization reagent remain in the packaging materials after sterilization and the residues affect on quality of an article; the gas sterilization has also problems of remaining gas, a poor efficiency caused by batch processing, and a bad influence on quality of an article; the ultraviolet sterilization has also a problem that the sterilization effect is insufficient, especially, ineffective to molds; the heat sterilization is in danger of exerting thermal deformation and thermal contraction on plastic packaging materials, and use of a heat resistant packaging material to suppress the thermal deformation and thermal contraction increases costs; the sterilization by plasma under vacuum has a problem of making a system expensive and also having a poor efficiency due to batch processing; and in the sterilization by plasma under normal pressure using an Rf power source, an electrode gap needs to be a few millimeters or less, therefore, objects for sterilization are restricted to those of a flat form like a film, and the sterilization effect has also been insufficient.
For the above reason, methods for sterilizing packaging materials (for example, plastic containers, paper containers, glass containers, films, sheets, or the like) without using germicides, moreover, without heating are under development. One of the sterilization methods uses high voltage pulses, for example, the sterilization method is provided the system with a power source for generating high voltage, a discharge side electrode for applying thereto the high voltage generated by the power source, and a ground side electrode placed to be opposed to the discharge side electrode. By this method, the object is sterilized by placing a packaging material as an object for sterilization between both electrodes at normal temperature and under normal pressure, and applying the high voltage pulses across the electrodes.
As an embodiment of a conventional high voltage pulse sterilization method, for example, as shown in FIG. 9-(a), a sterilizer A for containers is provided with a processing device comprising a power source B for generating high voltage, a discharge side electrode C for applying the high voltage generated by this power source B, a entrance and exit means (not illustrated) for putting this discharge side electrode C in and out of a packaging container D such as a PET bottle, and a ground side electrode E placed outside of the packaging container D, and is also provided with a gas supply means G for supplying a rare gas such as argon and helium into the packaging container D.
Moreover, the above-described power source B steps up the voltage inputted from the power source H through a Slidac I and a high voltage transformer J, and rectifies it by a full-wave rectification bridge K. Thereafter, the current value is decreased through a resistor L and a capacitor M is charged. The electric energy charged in the capacitor M is instantaneously discharged through a gap N, and is applied as a pulse voltage across the discharge side electrode C of the processing device F and the ground side electrode E, to sterilize a packaging material D by plasma generated by the high voltage pulses. In the figure, P is the resistor, and Q is the ground. Here, when a rare gas such as argon is supplied during discharge as shown in FIG. 9(b), breakdown voltage can be lowered, the plasma can be uniformly generated by the high voltage pulses, and favorable sterilization can be performed. Here, R is a dielectric provided inside of the ground side electrode E.
According to the above-described conventional high voltage pulse sterilization method, for example, in the case that the packaging material is a PET bottle or the like, the sterilization effect has been only about 1 to 2D [sterilization effect value D=−log (the survival number of bacteria/the initial number of bacteria)], and the sterilization has not been performed uniformly, and a practical sterilization effect has not been obtained.
The zealous studies repeated by the applicants of the present invention to solve the above problem have shown that the non-uniformity of the sterilization has been caused by the configuration that the discharge side electrode is in a shape of a round bar; strong high voltage pulse discharges are generated from the tip; and the strong high voltage pulse discharges are not effectively generated from the external wall of the round bar shape discharge side electrode, and the applicants have resulted in this invention.
Therefore, the first purpose of the present invention is to provide a method for sterilizing a packaging material by using high voltage pulses, in which non-uniformity is not caused to the sterilization effect and high voltage pulses are used in order to improve the sterilization effect, and the second purpose is to provide a sterilizer to be used therefor.